Austenitic steel of the cr-ni-mn group

ABSTRACT

D R A W I N G AUSTENITIC STEEL WITH HIGH STRENGTH AND OXIDATION RESISTANCE AT HIGH TEMPERATURE PREPARED BY ADDING TA, NB AND/ OR TI TO A STEEL BASICALLY COMPRISING CR, NI AND MN. FURTHER IMPROVEMENT IS HIGH TEMPERATURE IS STRENGTH IS ACHIEVED BY INCORPORATION OF B OR V IN ADDITION TO TA, NM AND/OR TI.

U nitcd States Patent 3,795,509 AUSTENITIC STEEL OF THE Cr-Ni-Mn GROUP Tohru Mimino, Kazuhisa Kinoshita, Takayuki Shinoda, and Isao Minegishi, Kanagawa-ken, Japan, assignors to Nippon Kokan Kabushiki Kaisha, Ootemachi, Japan Continuation-impart of abandoned application Ser. No. 774,092, Nov. 7, 1968. This application Jan. 8, 1971, Ser. No. 105,096

Claims priority, application Japan, Nov. 10, 1967, 42/71,952 Int. Cl. C22c 39/20 US. Cl. 75-128 A 5 Claims ABSTRACT OF THE DISCLOSURE Austenitic steel with high strength and oxidation resistance at high temperature prepared by adding Ta, Nb and/ or Ti to a steel basically comprising Cr, Ni and Mn. Further improvement in high temperature strength is achieved by incorporation of B or V in addition to Ta, Nm and/ or Ti.

CROSS REFERENCE TO RELATED APPLICATIONS The present application is a continuation-in-part of our copending application Ser. No. 774,092, filed Nov. 7, 1968 now abandoned.

BACKGROUND OF THE INVENTION At the present time, the steels being used in Japan where high strength at high temperatures is required are SUS 29 (similar to AISI 321), SUS 32 (similar to AISI 316) and SUS 28 (similar to AISI304L) and Japanese Industrial Standard SUS 27 (similar to USA. AISI304).

In the field where high strength and oxidation resisting property at high temperatures are required, a boiler will be taken for example. Recently, super-critical pressure boilers and large sized boilers have been Widely used at higher temperatures and higher pressures than heretofore. Materials for these constructions must necessarily have the required high degree of strength at high temperature. At present, for high temperature and high pressure, austenitic steel of the 18-8 group such as SUS 27 steel, SUS 28, SUS 29 and SUS 32 steel are much used. However, SUS 29 steel contains as much as 9 to 13% nickel and, not only is the nickel content high, but also the titanium content is more than 5 times greater in weight than the carbon content. Moreover as SUS 32 steel contains 2 to 3% molybdenum, high temperature strength can be obtained with these metals; however, they are expensive as materials. Although SUS 27 steel and SUS 28 steel are cheaper than said SUS 29 steel and SUS 32 steel, they are inferior in high temperature strength and therefore they cannot be used Where high temperature strength is especially required. Consequently, the expensive SUS 29 steel or SUS 32 steel are used. However, since boilers tend to be used at higher temperatures and in greater sizes, it is necessary that the materials to be used should be much cheaper as well as stronger than SUS 29 steel and SUS 32 steel. Such requirements are encountered not only in the field of boilers but also in other fields which need high temperature strength.

Normally, the Cr-Ni-Mn group is used with a great quantity of nitrogen in order to stabilize austenitic phase as well as a substantial quantity of costly additional elements such as molybdenum, titanium, vanadium, niobium, Wolfram and so forth, to heighten high temperature strength. Consequently, material costs will be higher and also the long-time creep rupture strength tends to be lower, on account of the great content of nitrogen.

3,7 95,509 Patented Mar. 5, 1974 The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram which compares in creep rupture strength the austenitic steels of the Cr-Ni-Mn Group in accordance with the present invention with the austenitic steel of the Cr-Ni-Mn Group containing none of the additional elements and SUS 27 steel, SUS 29 steel and SUS 32 steel which are used as high temperature and high pressure steels;

FIG. 2 is a diagram which compares in creep rupture strength the austenitic steels of the Cr-Ni-Mn Group with the austenitic steel of the Cr-Ni-Mn Group without boron in the additional elements and SUS 27 steel, SUS 29 steel and SUS 32 steel; and

FIG. 3 is a diagram which compares in creep rupture strength the austenitic steels of the Cr-Ni-Mn Group with the austenitic steel of Cr-Ni-Mn Group without vanadium in the additional elements and SUS 27 steel, SUS 29 steel and SUS 32 steel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With the objective of improvement of austenitic steels of 18-8 Group this invention replaces a part of the nickel with manganese Without adding said harmful nitrogen and adds a small quantity of titanium, niobium and tantalum to lower the metal alloy element costs as well as to obtain a stable austenitic composition. Thus, this invention has succeeded in achieving high temperature resistant heatproof steel which is economical, and free of problems with respect to its plasticity and weldability.

In other words, this invention relates to modified austenitic steels, the basic composition of which is' Cr-Ni-Mn; the chemical composition is essentially as follows:

Chromium: 15.0 to 21.0% (preferably 17.0 to 19.0%)

Nickel: 4.0 to 15.0% (preferably 5.0 to 8.0%)

Manganese: 4.0 to 12.0% (preferably 6.0 to 10.0%)

Canbon: 0.03 to 0.30%

Titanium: 0.001 to 0.51% (preferably 0.01 to 0.2%)

Niobium and/or tantalum: 0.001 to 0.50% (preferably The balance is composed of iron with some impurity. In other modifications, boron or vanadium are added, with the niobium and/or tantalum.

The reason why in addition to the elements Cr, Ni and Mn shown in the above mentioned compositions titanium, niobium and/ or tantalum are included is that if titanium only or niobium and/or tantalum only. are added, it is impossible to avoid the coalescence of carbide precipitating by the mutual action of carbide, thereby lowering the long time strength. However, if there coexist even a small quantity of titanium and niobium and/ or tantalum at the same time, they act on one another to avoid the coalescence so that a uniform and fine carbide distribution can be efficiently effected. In this connection, it is desirable that about 50% is ratio of the total number of atoms of said titanium, niobium and/ or tantalum to that of carbon. This contributes to a remarkable effect to improve the strength by distributing the precipitated carbide of chromium around carbides of titanium, niobium and tantalum.

The reason for the adoption of the above mentioned composition range is that if chromium is less than 15.0%, the oxidation resistance will be poor and if it is more than 21.0%, the delta will appear as a consequence of balance with other elements so that a single austenitic phase is difiicult to obtain, so around 17.0 to 19.0% is optimal. If nickel is below 4.0%, it is impossible to obtain the single austenitic phase and if more than 15.0%, it will be economically disadvantageous; around 5.0 to 8.0% is preferable. Further, as to manganese if it is less than 4.0%, the delta phase appears in case nickel is low, so that it is diflicult to obtain the single austenitic phase; besides the strength at high temperature'is reduced. Conversely, if it is higher than 12.0%, it is disadvantageous owing to the extreme reduction in said strength. If the manganese content is too high, the delta phase is easy to appear; the most suitable range is around 6.0 to 10.0%. If the titanium content is more than 0.5%, the titanium carbide becomes too high, they cause coalescence and if used for a long time at high temperature the strength is reduced by the coarsening of carbide particles, and if it is below 0.001%, the titanium carbide quantity is too low so that it does not contribute to improvement of the strength. So, the most desirable range is around 0.01 to 0.20%. In the case where niobium and tantalum are below 0.001%, since niobium and/or tantalum carbide quantities are lower, no elfect thereof will appear. On the other hand, if they are more than 0.50%, there is too much precipitated carbide of niobium and tantalum, and they cause coalescence so that it lowers the high temperature strength; the optimal range is around 0.05 to. 0.30%. Concerning carbon, if it is below 0.03%, there is a little quantity of precipitated carbide and if more than 0.3%, there is too much, conversely, causing to lower the long time creep rupture strength.

Table I shows chemical compositions embodying this invention. 1

Table II shows the result of creep rupture strength tests performed on the above mentioned 4 kinds of steels after they are melted and subjected to the appropriate conditions at each of the temperatures of 600 C., 650 C. and 700 C., respectively, during 10 hr. and 10 hr.

TABLE 11 Unit, kgJmm. at-

Number 600 C 650 C. 700-C.

For reference, Table HI shows a chemical composition of austenitic steel of the Cr-Ni-Mn Group which contains no added titanium, niobium or tantalum; Table IV shows creep rupture strength at each of the temperatures, 600 0., 650 C. and 700 C. during hr. and 10 hr.

TABLE III Unit, weight percent at-' Number 0 Si Mn Ni of Ti Nb v On the other hand, Table V shows the result of creep rupture strength tests performed on the conventional SUS 27. steel, SUS 29 steel, and SUS 32 steel at each of the temperatures, 600 C., 650 C. and 700 C., during 10 hr. and 10 hr.

TABLE V Unit, kgjmrn! at- 10 hrs. 10 hrs. 10 hrs. 10 hrs. 10 hrs. 10 hrs.

SUS 27 15.1 10.8 10.2 7.3 7.0 4.9 SUS 29-.. 25. 5 18. 5 17.5 11. 6 12. 0 7. 4 SUS 32... 24. 5 18. 0 16. 2 11. 5 10.5 7. 0

A comparison of creep rupture strength is shown in FIG. 1 which diagrams the results in Tables II, IV and V; it is evident that the creep rupture strength of No. 5 (Cr-Ni-Mn steel containing no titanium, niobium or tantalum) is equal or inferior to the presently used SUS steel 27, SUS steel 29 and SUS steel 32, while those of No. 1, No. 2, No. 3 and No. 4 of this invention have more than twice as great a creep rupture strength as SUS 27 steel, and about 1.5 times as much as that of SUS steel 32.

In this way, this invention improves the unsatisfactory high temperature strength of the conventional steel. More over, the high temperature strength can be even further improved by adding a small quantity of boron and/or vanadium to the above-mentioned addition elements namely titanium, niobium and/ or tantalum. The quantity of boron should be 0.0001 to 0.050%, preferably 0.001 to 0.02%, and, of vanadium, 0.001 to 1.0%, preferably 0.01 to 0.1%.

As mentioned above, the action of this boron addition can provide distribution of a uniform and fine carbide owing to multiplying action of titanium and tantalum. When chromium carbide is precipitated and distributed around these titanium, niobium and tantalum carbides, the presence of boron avoids chromium carbide from coalescing, thereby distributing it uniformly, also boron itself combines with carbon to contribute to improvement of high temperature strength. In this connection, boron is used within the above composition range, because if it 7 is below 0.0001%, its effect cannot be perceived and if more than 0.050%, difiiculties arise with respect to plasticity and processing.

The action of vanadium is the same as that of boron. The reason why it should be present within the above composition range is that if it is below 0.001%, the effect cannot be perceived, and if more than 1.0%, too much vanadium carbide will form and it will coalesce itself, lowering the strength for long-time use; moreover, it is not economically advantageous to add too much.

. Hereunder, an explanation will be given as to the embodiment: Table VI shows the chemical composition with addition of boron.

TABLE VI Unit, weight; percent a.t

Number 0 Si Mn Ni Cr Ti Nb-l-Ta B The above mentioned four kinds of steel were solutiontreated followed by water-quenching after which they were creep-rupture-tested at each oi the temperature 600 C., 650 C. and 700 C. during 10 hr. and 10 hr., the

6 The comparative test results are given in Table IX and No. 1 of Table II (heat resisting steel of the first modification containing titanium, niobium and/ or tantalum and no boron) and Table V (creep rupture strengths of SUS 27 steel, SUS 29 steel and SUS 32 steel) which are diaresults are given in Table VII.

grammed in FIG. 3. It is found that the rupture strengths of No. 10, No. 11, No. 12 and No. 13 of the third modification are around 1.2 times as much as No. 1 and 2 to 3 times as much as SUS 27 steel, and 1.4 to 2 times as much TA VII as SUS 29 steel or SUS 32 steel. Consequently, it is noted as in the addition of boron that addition of vanadium o a a with addition of titanium and niobium and/or tantalum 650 700 effectively produce higher temperature strength. Thus, the Number 10 1118. 10 1115. 1O3I1IS. 10 hrs. 10 hrs. 10111'5. present invention ubstantially improves the tem- 3L6 2m 22 2M 1&6 15 perature strength which was unsatisfactory with the con- 7 7 ventional steel. Moreover, it is oxidation resistant and 33:3 32:3 23;? lg: {:12 free of difficulty, with respect to plasticity and weldability. As to its price, part of the costly nickel is replaced with cheaper manganese, and only small quantities of 20 titanium, niobium and/or tantalum and further small quantities of boron or vanadium are added, so that lowering of price results. Consequently, it can be utilized in all The results of the comparative tests run on these steels the fields Where high Strength and Oxidation resistance are listed in Table VII and No. 1 of Table II (heat reare necessary under high temperature working conditions, sisting steel of the first embodiment containing titanium, 25 for example Where it is utilized as P p material for i i and/or tantalum d no boron) and bl V thermoelectric power boilers which are becoming larger (creep rupture strength of SUS 27 steel, SUS 29 steel in size and are Operated at higher temperatures and and SUS 32 steel) which are diagrammed in FIG. 2; it higher Pressures In this y it is expected that the P is found that the creep rupture trengths of No 6 NO, 6I'lt invention substantially reduce the COSt Of such 7, No. 8 and No. 9 of the second embodiment are around 30 equipment as as enable it to withstanfl high 1.3 times as great as No. 1 and 2.5 to 3 times as great as Perature and hlgh pmsf'uresr i have the SUS 27 steel, and 1.5 to 2 times as great as SUS 29 steel excellent CfiGFt Of reducmg the elect Pnceor SUS 32 steel. Consequently, it is evident that the pres- 1 fig ig gfigg g s g 3 233 g ig g ence of boron m ajddltlon to mzfmmm and rilobmm and/ 35 found that there are certain relationships among the additantalum eficenvely results greater hlgh tempfla' tives to the basic steel which must lie in certain regions tum strength for opimization of high temperature strength of the steels. The following Table VI'H shows the chemical composl- The first of these relations which may be termed non of another group of embodnnents wherem vanadium ratios is given in the Seventh column of the f ll win is added instead of boron. 40 T bl X,

TABLE X Composition of test steels (percent) of- Tt+1l2 (Nb-i-Ta) C Si Mn N1 01' T1 Nb+Ta C 0.18 0.57 8.65 5. 98 17.84 Trace Trace 0 0.20 0.59 8.10 0.13 17.77 0.048 0. 000 0.38 0.18 0.01 8.12 0.34 18.15 0.000 0.11 0.04 0. 20 0. 62 8.10 0.34 18.23 0.000 0. 20 0. 80 0.20 0.00 8.10 6.08 17.85 0.21 0.21 1.0 0.19 0. 54 7.84 0.02 16.68 0.48 0. 03 1.2

TABLE VIII Unit, weight percent at Cr Ni Tl Nb+Ta V The above four kinds of steel were melted and treated under the usual conditions; creep rupture strength tests were then performed at each of the temperature, 600 C., 650 C., and 700 C. during 10 hr. and 10 hr.; the results are shown in the following Table IX.

TABLE IX Unit, kgJnnn. at-

Number 10 hrs. 10 hrs. 10 hrs. 10 hrs. 10 hrs. 10 hrs.

The quantities listed in Table X, including those in the key ratio are in percent of total weight.

The creep rupture strength on the steels of Table X are given in Table XI.

As can be seen from the results in Table XI, the highest creep rupture strengths are attained when the first key ratio lies between 0.38 and 1.6. Substantial improvement of steels are achieved when the key ratio lies between 0.2 and 3.5.

It is somewhat surprising that there is a maximum in the relationship between the high temperature strength of steels and the quantity of titanium, niobium and tan- 7 talum added. It is our belief that the key ratio stems from the atomic ratios of the added titanium, niobium and tantalum to the carbon percent of the steel. The additives form carbides with the carbon. Only half of the niobium content is taken because the atomic weight of niobium is roughly double that of titanium. By the same reasoning only one quarter of the tantalum should be counted in preparing the key ratio; however, tantalum and niobium are difiicult to distinguish in the usual types of analyses 'of these elements, and to simplify matters only half of the tantalum content is taken instead of one quarter.

We have also found that excellent creep rupture strength is achieved when the titanium and niobium and tantalum contents when expressed as a second key ratio fall within certain limits. The second key ratio is shown in Table XII.

We claim:

1 Austenitic heat-proof steel of the Cr-Ni-Mn group, comprising essentially 15.0-21.0% Cr; 4.0-15% Ni; 4.0- 12.0% Mn; (Ll-1.0% Si; 0.03-0.30% C; 0.00l0.50%'Ti; and 0.00010.50% of a metal chosen from the group consisting of Nb and Ta, the remainder being Fe, wherein a first key ratio,

Ti+ /2 (Nb+Ta) where quantities are given in weight percent, lies between 0.2 and 3.5 and a second key ratio Ti+ /z (Nb+Ta) It should be noted that the first key ratio for the compositions listed in Table XII lies between 0.73 and 1.16, values which are well within the boundaries proposed above for the first key ratio.

The results of the creep rupture strength on the above test steels are shown in Table XIII.

TABLE XIII 1000 hr. creep rupture strength at 700 C. (kg/mm?) No. 31 8.8 No. 32 9.0 No. 33 10.5 No. 34 12.5 No. 35 12.9 No. 36 11.0 No. 37 10.2 No. 38 7.3

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

where quantities are given in weight percent, lies between 0.15 and 0.85.

2. Austenitic steel as defined in claim 1, wherein said Cr content is 17.0-19.0%, said Ni content is 5.08.0%, said Mn content is 6.0 10.0%, said Si content is 0.1-1.0%, said C content is 0.03-0.30%, said Ti content is 0.01 0.20%, and the content of said metal chosen from the group consisting of Nb and Ta is ODS-0.30%, the remainder being iron.

3. Austenitic steel, as defined in claim 1, wherein the content of said metal chosen from the group consisting of Nb and Ta lies between 0.001 and 0.50%.

'4. 'Austenitic steel as defined in claim 1, further comprising .000.05 B.

5. Austenitic steel as defined in claim 1, further comprising .001 to .1% of V.

References Cited UNITED STATES PATENTS 2,190,486 2/1940 Schafmeister --128 G 3,112,195 11/1963 Souresny 75-128 G 3,401,036 2/1968 Dulis 75-128 R 3,607,239 9/ 1971 Mimino et al 75--128 G HYLAND BIZOT, Primary Examiner US. Cl. X.R.

75l28 G, 128 T 

